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Comparison of equations and estimation procedures of lactation curves in Angus, Brown Swiss and their crosses.

Abstract

Milk production data (272 lactations) collected weekly by weigh-suckle-weigh techniques up to 180 d, from a diallel crossbreeding experiment at the University of Chapingo, Mexico, using Angus (A) and Brown Swiss (B) breeds were recorded. The purpose was to compare eight equations and two estimation methods of lactation curves in A, B, and reciprocal-cross cows. The following equations were evaluated for each lactation record: Yt = atbe‘ct (I); Yt = ae'ct (II); Yt = t/aect (III), Yt = ae(bt+ct"' (IV); Yt = a + bt + ct2 (V); Yt = a + bt (VI); Yt = a - bt - ae'c{ (VII), Y{ = t/(a + bt + ct2) (VIII). Parameters of the first four equations were estimated by linear (L) and nonlinear (N) least squares regression. Residual mean squares (RMS) were used as a criterion to evaluate curve fitting. Ranks for the eight equations were compared using Friedman's test Generally the difference in RMS between L and N were not significant (P > .1), except for equation III, but always favored N, Equations I and VHI had the best fit to A lactation curves, whereas IV, I and V fitted better B and reciprocal-cross cows. The best equation was dependent on cow's genotype.